It is a conventional practice to encapsulate semiconductor elements such as transistors, ICs and LSIs with ceramic materials and the like for fabrication of semiconductor devices. Recently, semiconductor devices of resin-encapsulated type employing plastic materials have become dominant in view of costs and mass productivity thereof. Epoxy resin compositions are conventionally used for the resin encapsulating of the semiconductor devices of this type, offering good performance. However, technological innovation in the semiconductor field has led to a higher integration degree, a larger device size and a more minute interconnection pattern, while the current trend is toward reduction in package size and thickness. This demands further improvement in the reliability of an encapsulating material.
On the other hand, electronic components such as semiconductor devices should absolutely satisfy the flame retardance requirement of the UL94 V-0 standard. One conventional method of imparting flame retardance to a resin composition for semiconductor encapsulating is to add a brominated epoxy resin and antimony oxide to the resin composition.
However, there are two critical problems associated with the aforesaid flame retardance imparting technique.
A first problem is that antimony trioxide itself has a toxicity and the resin composition, when burnt, emits hydrogen bromide, bromine-based gases, antimony bromide and the like which are harmful to human body and equipment because of their toxicity and corrosiveness.
A second problem is that, if a semiconductor device fabricated by utilizing the aforesaid flame retardance imparting technique is allowed to stand in a high temperature atmosphere for a long period, liberated bromine corrodes aluminum interconnections formed on a semiconductor element of the device. This results in breakdown of the semiconductor device, presenting the problem of reduction in the high temperature reliability of the device.
Proposed as one approach to the aforesaid problems is addition of an inorganic flame retardant of a halogen- and antimony-free metal hydroxide. However, this approach requires to use the metal hydroxide in a large amount (e.g., 40% by weight or greater), thereby posing new problemns.
A first problem is that the semiconductor device is liable to swell and crack during a soldering operation. In recant years, a surface mounting method has become dominant as a method for the mounting of the semiconductor device. The soldering operation employs a solder dipping process, an infrared reflow process, a vapor phase reflow process or the like. Whichever process is employed, the semiconductor device is subjected to a high temperature (typically, 215.degree. C. to 260.degree. C.). In the case of a semiconductor device encapsulated with a conventional resin composition containing the metal hydroxide, abrupt evaporation of water absorbed in the metal hydroxide causes the semiconductor device to swell and crack, because the metal hydroxide has a high water content. That is, the problem is associated with reduction in so-called soldering resistance.
A second problem is associated with moisture resistance reliability. That is, the performance of the semiconductor device is liable to deteriorate in a high temperature and high humidity atmosphere at a temperature of 80.degree. C. to 200.degree. C. and a relative humidity of 70% to 100%. Where the semiconductor device includes highly exothermic semiconductor elements or is mounted adjacent to an engine of an automobile, dehydration of the metal hydroxide occurs in the semiconductor device during prolonged use, thereby deteriorating the moisture resistance reliability.
The conventional flame retardance technique involves the aforesaid problems and, hence, there is a strong demand for development of a new flame retardance technique which uses a safe material free from emission of harmful gases when the material is burnt, and causes neither swelling nor cracking in a semiconductor device due to dehydration of the metal hydroxide during the soldering operation, nor the corrosion of aluminum interconnections on a semiconductor element of the device nor the deterioration in moisture resistance reliability even if the semiconductor device is allowed to stand in a high temperature and high humidity atmosphere for a long period. To solve the aforesaid problems, the inventors of the present invention previously proposed to employ a thermosetting resin composition for semiconductor encapsulating which contains a thermosetting resin, a hardening agent, a metal hydroxide and a metal oxide, or a compound metal hydroxide consisting of the metal hydroxide and the metal oxide (Japanese Patent Application No. HEI 7-507466). The use of the thermosetting resin composition for semiconductor encapsulating indeed improves the flame retardance and the moisture resistance reliability, but poses a new problem. When a semiconductor package having a reduced thickness (which is a recent trend) is molded by transfer molding or the like, the fluidity of the resin composition as the encapsulating material is reduced thereby to deform gold wires of a semiconductor device. That is, the problem is associated with remarkable deterioration of the moldability.
In view of the foregoing, it is an object of the present invention to provide a resin composition for semiconductor encapsulating which is safe and superior in moisture resistance reliability, flame retardance and moldability, a semiconductor device using such a resin composition, and a process for fabricating such a semiconductor device.